Receivers that are intended for the detection of radio frequency signals, for example of radar type, must be able to monitor wide frequency bands notably in the microwave domain. They must be able to detect, for example, radar signal pulses. These pulses may exhibit “chirp” type or phase code type modulations, these modulations being used by the radar to compress the pulse on reception. The pulses may also be unmodulated, other than by the all-or-nothing modulation defining the pulse. The role of these receivers is, among other things, to estimate the frequency or frequencies of the received signal or signals, and their amplitude.
Although digital processing operations are normally involved in the architecture of these receivers, the current solutions for estimating the frequencies of the received signals, hereinafter in the description designated by the acronym IFM (Instantaneous Frequency Measurement), are mostly based on analog techniques.
As an example, one existing solution for an instantaneous measurement of the frequency of the received signal is based on the creation of a regime of standing waves in a propagation line attacked at one of its ends by the signal and at the other by the delayed signal. The periodicity of the nodes and of the antinodes gives a rough measurement of the frequency of the received signal. The measurement of the positions of the nodes and of the antinodes distributed along this line gives a fine measurement of the frequency of the received signal. This type of IFM is called a frequency meter with spatial sampling.
Another solution of the prior art, which is very widely used, is based on self-correlators, or phase meters. The principle in this case is to directly measure the phase difference φ induced by a delay line and deduce the frequency therefrom. Several stages, placed in parallel, are generally needed to ensure the desired frequency band and accuracy.
The receivers that implement these solutions use analog functions and are therefore subject to drifts such as delay variations according to temperature, or according to measurement, level or phase shift imperfections. This results in a bulky and very costly architecture.
More recently, a French patent application relating to a frequency measurement wideband digital receiver filed under the number 06/01205 describes, unlike the previous two examples, a way of digitizing the signal at the input, and of performing all the processing operations in digital mode. The digitization is performed at a sampling frequency far below the Shannon criterion. This is reflected in an ambiguous frequency measurement Fmeasure, also called fine frequency hereinafter in the description, said frequency being able to be described by the following expression:Fmeasure=±(Freal−j×Fe)+δF  (1)
in which Freal is the real frequency of the received signal, j a positive integer, Fe the sampling frequency of the system and δF the measurement error due mainly to the signal-to-noise ratio.
To resolve this ambiguity, N measurement channels are used in parallel, with offset sampling frequencies. The N ambiguous frequency measurements are associated to resolve said ambiguity and obtain a measurement of the real frequency of the incident signal. This measurement is called final frequency hereinafter in the description.
When an instantaneous frequency measurement device is used, said device is usually coupled with a second device whose function is to measure the amplitude. This makes it possible notably to detect the presence or absence of signals and their associated characteristics. This is because these IFMs operate in a wide dynamic range. This is achieved through the use of a limiting amplifier at the head of the device and eliminates any level information. This type of IFM therefore does not have any inherent capability of detecting the envelope of the received signal pulses. This is why it is usually associated with a level measurement device, for example of DVLA (Detector Logarithm Video Amplifier) type. The latter supplies additional information on the amplitude of the received signal or signals and makes it possible notably to validate or invalidate the frequency measurement produced by the IFM and to estimate the envelope of the received signals. Adding such a device de facto induces a greater size and cost.